Web Animations CSS: Fix crash when animating viewport units

Source/core/animation/AnimatableLength.cpp

Index: Source/core/animation/AnimatableLength.cpp
diff --git a/Source/core/animation/AnimatableLength.cpp b/Source/core/animation/AnimatableLength.cpp
index 886ff9b546d44548bde426f1df259d5b9695ceee..f89477c6f65ac7e0f094e94435c5c45d495a1f6b 100644
--- a/Source/core/animation/AnimatableLength.cpp
+++ b/Source/core/animation/AnimatableLength.cpp
@@ -97,6 +97,10 @@ PassRefPtr<AnimatableValue> AnimatableLength::interpolateTo(const AnimatableValu
     if (type != UnitTypeCalc)
         return AnimatableLength::create(blend(m_number, length->m_number, fraction), type);
 
+    // FIXME(crbug.com/168840): Support for viewport units in calc needs to be added before we can blend them with other units.
+    if (isViewportUnit() || length->isViewportUnit())
+        return defaultInterpolateTo(this, value, fraction);
+
     return AnimatableLength::create(scale(1 - fraction).get(), length->scale(fraction).get());
 }